How many mm in an inch?

25.4 exactly. Since the 1959 international agreement on the yard and pound, the inch is defined as exactly 25.4 millimetres — it is a definition, not a measurement, so it is exact to any number of decimal places and never changes.

What is the difference between metric and imperial conversions?

Imperial length and mass units are defined in terms of metric ones, so converting imperial to metric is exact. Converting metric to imperial usually gives a repeating decimal — 1 mm is 0.03937007874 in with no end — so those factors are approximations quoted to a chosen precision.

How do I convert Celsius to Fahrenheit?

Multiply by 9/5 and add 32: °F = (°C × 1.8) + 32. So 100 °C is 212 °F. This is not a pure factor because the two scales have different zero points — which is why temperature conversions cannot be done by multiplying alone.

How do I convert a temperature difference?

Multiply by 9/5 only — do not add 32. A 10 °C rise is an 18 °F rise. The offset applies only to absolute temperatures, and applying it to a difference is one of the most common conversion errors in heat transfer and thermal expansion work.

How many psi in a bar?

14.50377377. A bar is exactly 100 000 Pa and a psi is exactly 6894.757293168 Pa, so the conversion follows from definitions. Note that 1 atmosphere is 14.6959 psi — the familiar 14.7 figure belongs to the standard atmosphere, not to the bar.

How do I convert kg to pounds?

Multiply by 2.204622622. The reverse factor, 0.45359237 kg per pound, is the exact definition. Note the distinction between mass and force: a kilogram converts to pound-mass this way, but a kilogram-force converts to pound-force with the same factor because both include gravity.

Conversion Chart

Conversion factors for 84 engineering unit pairs, grouped by quantity, with each factor marked exact or approximate — which is the distinction that decides how many digits you can trust.

Data verified 2026-09-29 · based on NIST SP 811, 2008 edition

Quick Answer

1 inch = 25.4 mm exactly and 1 lb = 0.45359237 kg exactly — these are definitions, not measurements, so they never change and can be quoted to any precision. Only about half the factors in this table are exact. The rest, including BTU and horsepower, have more than one historical definition, and temperature cannot be converted by a factor at all — it needs an offset.

Find a conversion

Unit Conversion Factors

Conversion^[2]	Quantity^[2]	Multiply by^[2]	Exact?^[1]	Note^[2]
inch → millimetre #	Length	25.4	Exact	The definition of the inch since 1959 — 1 in = 25.4 mm exactly
inch → centimetre #	Length	2.54	Exact	Same definition, expressed in cm
foot → metre #	Length	0.3048	Exact	Exactly 12 × 25.4 mm
yard → metre #	Length	0.9144	Exact	Exactly 36 × 25.4 mm
mile → kilometre #	Length	1.609344	Exact	Exactly 1760 × 0.9144 m
mil (thou) → millimetre #	Length	0.0254	Exact	One thousandth of an inch — the machinist's mil
nautical mile → metre #	Length	1852	Exact	Defined exactly; not derived from the statute mile
millimetre → inch #	Length	0.03937007874	Approx.	Reciprocal of 25.4, quoted to 10 significant figures
metre → foot #	Length	3.280839895	Approx.	Reciprocal of 0.3048
kilometre → mile #	Length	0.6213711922	Approx.	Reciprocal of 1.609344
centimetre → inch #	Length	0.3937007874	Approx.	Reciprocal of 2.54
micrometre → microinch #	Length	39.37007874	Approx.	The conversion behind surface finish specifications
Ångström → nanometre #	Length	0.1	Exact	Defined as exactly 10⁻¹⁰ m
square inch → square centimetre #	Area	6.4516	Exact	25.4² exactly
square foot → square metre #	Area	0.09290304	Exact	0.3048² exactly
square yard → square metre #	Area	0.83612736	Exact	0.9144² exactly
acre → square metre #	Area	4046.8564224	Exact	Exactly 4840 square yards
acre → square foot #	Area	43560	Exact	By definition
square mile → square kilometre #	Area	2.589988110336	Exact	1.609344² exactly
hectare → square metre #	Area	10000	Exact	By definition; 100 ares
square millimetre → square inch #	Area	0.0015500031	Approx.	Reciprocal of 645.16
cubic inch → cubic centimetre #	Volume	16.387064	Exact	25.4³ exactly
cubic foot → litre #	Volume	28.316846592	Exact	0.3048³ × 1000 exactly
US gallon → litre #	Volume	3.785411784	Exact	By definition of the US liquid gallon
Imperial gallon → litre #	Volume	4.54609	Exact	By definition — 20% larger than the US gallon
US gallon → cubic inch #	Volume	231	Exact	By definition
US quart → litre #	Volume	0.946352946	Approx.	One quarter of a US gallon
US fluid ounce → millilitre #	Volume	29.5735295625	Approx.	One 128th of a US gallon
US barrel (oil) → litre #	Volume	158.987294928	Approx.	Exactly 42 US gallons
litre → US gallon #	Volume	0.2641720524	Approx.	Reciprocal of 3.785411784
cubic metre → cubic foot #	Volume	35.31466672	Approx.	Reciprocal of 0.028316846592
pound (avoirdupois) → kilogram #	Mass	0.45359237	Exact	The definition of the pound since 1959
ounce (avoirdupois) → gram #	Mass	28.349523125	Exact	Exactly 1/16 pound
tonne (metric ton) → kilogram #	Mass	1000	Exact	By definition
short ton (US) → kilogram #	Mass	907.18474	Exact	Exactly 2000 pounds
long ton (UK) → kilogram #	Mass	1016.0469088	Exact	Exactly 2240 pounds
grain → milligram #	Mass	64.79891	Exact	Exactly 1/7000 pound
slug → kilogram #	Mass	14.59390294	Approx.	The imperial unit of mass in the ft-lb-s system
kilogram → pound #	Mass	2.204622622	Approx.	Reciprocal of 0.45359237
pound-force → newton #	Force	4.4482216152605	Exact	Exactly 0.45359237 × 9.80665
kilogram-force → newton #	Force	9.80665	Exact	Defined by standard gravity
kip → kilonewton #	Force	4.4482216152605	Approx.	Exactly 1000 pounds-force
newton → pound-force #	Force	0.2248089431	Approx.	Reciprocal of 4.4482216152605
newton → kilogram-force #	Force	0.1019716213	Approx.	Reciprocal of 9.80665
ton-force (short) → kilonewton #	Force	8.896443230521	Approx.	Exactly 2000 pounds-force
pound per sq inch (psi) → kilopascal #	Pressure	6.894757293168	Exact	Reciprocal of the kPa-to-psi definition
bar → pound per sq inch (psi) #	Pressure	14.50377377	Approx.	From 1 bar = 100 000 Pa exactly
megapascal → pound per sq inch (psi) #	Pressure	145.0377377	Approx.	From 1 MPa = 10⁶ Pa exactly
atmosphere → kilopascal #	Pressure	101.325	Exact	The standard atmosphere, defined exactly
kilogram-force per sq cm → pound per sq inch (psi) #	Pressure	14.22334331	Approx.	From 1 kgf/cm² = 98 066.5 Pa exactly
inch of mercury (32 °F) → kilopascal #	Pressure	3.386389	Approx.	Conventional value used for barometers and altimeters
millimetre of mercury → pascal #	Pressure	133.322387415	Approx.	Conventional value at 0 °C
pound per sq inch (psi) → bar #	Pressure	0.06894757293	Approx.	Reciprocal of 14.50377377
British thermal unit (IT) → joule #	Energy	1055.05585262	Approx.	The International Table BTU — other BTU definitions exist
calorie (thermochemical) → joule #	Energy	4.184	Exact	Defined exactly; the 15 °C calorie differs
kilowatt-hour → megajoule #	Energy	3.6	Exact	By definition
foot pound-force → joule #	Energy	1.35581794833140	Exact	Exactly 0.3048 × 4.4482216152605
electronvolt → joule #	Energy	1.602176634e-19	Exact	Exact by the 2019 SI redefinition
therm → megajoule #	Energy	105.505585262	Approx.	Exactly 100 000 BTU (IT)
horsepower (mechanical) → kilowatt #	Power	0.7456998716	Approx.	Exactly 550 ft·lbf/s — other horsepower definitions exist
horsepower (metric) → kilowatt #	Power	0.73549875	Approx.	Exactly 75 kgf·m/s — about 1.4% smaller than mechanical hp
ton of refrigeration → kilowatt #	Power	3.516852842	Approx.	Exactly 12 000 BTU/h
BTU per hour → watt #	Power	0.2930710702	Approx.	From the IT BTU
kilowatt → horsepower (mechanical) #	Power	1.34102209	Approx.	Reciprocal of 0.7456998716
mile per hour → kilometre per hour #	Speed	1.609344	Exact	Exactly the mile-to-km factor
foot per second → metre per second #	Speed	0.3048	Exact	Exactly the foot-to-metre factor
knot → kilometre per hour #	Speed	1.852	Exact	One nautical mile per hour, defined exactly
surface feet per minute (SFM) → metre per minute #	Speed	0.3048	Exact	The conversion behind cutting speed tables
metre per second → foot per minute #	Speed	196.8503937	Approx.	Reciprocal of 0.00508
US gallon per minute (gpm) → litre per minute #	Flow	3.785411784	Exact	Exactly the US gallon
cubic foot per minute (cfm) → cubic metre per hour #	Flow	1.699010796	Approx.	60 × 0.028316846592
cubic foot per minute (cfm) → litre per second #	Flow	0.4719474432	Approx.	From the cubic foot definition
US gallon per minute (gpm) → cubic metre per hour #	Flow	0.2271247070	Approx.	From the US gallon definition
pound-force foot → newton metre #	Torque	1.35581794833140	Exact	Exactly 0.3048 × 4.4482216152605
pound-force inch → newton metre #	Torque	0.1129848290	Exact	One twelfth of the ft·lbf value
kilogram-force metre → newton metre #	Torque	9.80665	Exact	Defined by standard gravity
newton metre → pound-force foot #	Torque	0.7375621493	Approx.	Reciprocal of 1.35581794833140
degree → radian #	Angle	0.01745329252	Approx.	π/180 — irrational, so never exact
radian → degree #	Angle	57.29577951	Approx.	180/π
arcminute → degree #	Angle	0.01666666667	Approx.	One sixtieth of a degree
arcsecond → degree #	Angle	0.0002777777778	Approx.	One 3600th of a degree
gradian → degree #	Angle	0.9	Exact	By definition — 400 gradians to a full circle
hertz → revolution per minute #	Frequency	60	Exact	By definition
revolution per minute → hertz #	Frequency	0.01666666667	Approx.	Reciprocal of 60
foot-candle → lux #	Illumination	10.76391042	Approx.	One lumen per square foot
lux → foot-candle #	Illumination	0.09290304	Exact	Exactly the square-foot-to-square-metre factor
pound per cubic inch → gram per cubic centimetre #	Density	27.67990471	Approx.	0.45359237 / 16.387064
pound per cubic foot → kilogram per cubic metre #	Density	16.01846337	Approx.	0.45359237 / 0.028316846592
BTU per hour per sq ft per °F → watt per sq m per K #	Heat transfer	5.678263341	Approx.	The film coefficient conversion
watt per sq m per K → BTU per hour per sq ft per °F #	Heat transfer	0.1761101838	Approx.	Reciprocal of 5.678263341
BTU per hour per ft per °F → watt per metre per K #	Thermal conductivity	1.730735	Approx.	The k-value conversion
thousand pound per sq inch (ksi) → megapascal #	Stress	6.894757293	Approx.	Exactly 1000 psi
megapascal → thousand pound per sq inch (ksi) #	Stress	0.1450377377	Approx.	Reciprocal of 6.894757293

The Exact column is the important one. Since the 1959 international agreement on the yard and pound, every US customary and imperial length and mass unit is defined in terms of SI exactly — so 1 in = 25.4 mm and 1 lb = 0.45359237 kg to any number of figures, and a conversion built on them stays exact. Factors marked approximate are either reciprocals (which are usually irrational and are quoted to 10 significant figures here) or conventionally agreed values with genuinely limited precision, such as the inch of mercury used for barometers.

Multiply by the factor to go from the From column to the To column. To go the other way, divide by it rather than multiplying by the reciprocal — for exact factors the two are identical, but for approximate ones the reciprocal shown is rounded and dividing is more accurate. Temperature is not in this table because it is the one conversion that is not a simple factor.

Temperature Is Not a Factor Conversion

Every other quantity in the table converts by multiplying, because every other unit scale starts at zero. Temperature scales do not — 0 °C and 0 °F are different physical temperatures, so a conversion needs both a multiplier and an offset.

°F = (°C × 9/5) + 32
°C = (°F − 32) × 5/9
K = °C + 273.15
°R = °F + 459.67
°R = K × 9/5

This is where the common error lives. Converting a difference in temperature — a thermal expansion calculation, a heat load, a specific heat — uses only the multiplier 9/5 or 5/9 and no offset. Converting an absolute temperature uses both. Applying the +32 to a temperature difference is a large error, and it is easy to make because the two calculations look almost identical.

The absolute zero points are worth having to hand: −273.15 °C, −459.67 °F and 0 K are the same temperature. Note also that the rankine degree and the fahrenheit degree are the same size, as are the kelvin and the celsius degree — only the zero points differ.

Which Unit Does the Formula Want?

Conversion errors in engineering are rarely arithmetic mistakes. They are unit mistakes — the formula wanted metres and got millimetres, or wanted pound-mass and got pound-force.

The three that cause the most trouble are these. Mass against force: the pound is a mass and the pound-force is a force, and they are numerically equal only under standard gravity. A weight in pounds becomes a force in pounds-force only at g = 9.80665 m/s². Metres against millimetres: a stress in N/mm² is a megapascal, but a length in mm fed into a formula expecting metres gives an answer a thousand times out — and in deflection formulas, where the length is cubed, that becomes a factor of a billion. Kilogram-force against newton: kgf survives on gauges and in older catalogues and is 9.80665 N, so treating it as a newton understates a load by a factor of ten.

The defence is not to remember the factors — it is to carry units through the calculation and check that the answer comes out in the unit you expected. A dimension check catches essentially all of these, and it costs nothing.

Frequently Asked Questions

How many mm in an inch?: 25.4 exactly. Since the 1959 international agreement on the yard and pound, the inch is defined as exactly 25.4 millimetres — it is a definition, not a measurement, so it is exact to any number of decimal places and never changes.
What is the difference between metric and imperial conversions?: Imperial length and mass units are defined in terms of metric ones, so converting imperial to metric is exact. Converting metric to imperial usually gives a repeating decimal — 1 mm is 0.03937007874 in with no end — so those factors are approximations quoted to a chosen precision.
How do I convert Celsius to Fahrenheit?: Multiply by 9/5 and add 32: °F = (°C × 1.8) + 32. So 100 °C is 212 °F. This is not a pure factor because the two scales have different zero points — which is why temperature conversions cannot be done by multiplying alone.
How do I convert a temperature difference?: Multiply by 9/5 only — do not add 32. A 10 °C rise is an 18 °F rise. The offset applies only to absolute temperatures, and applying it to a difference is one of the most common conversion errors in heat transfer and thermal expansion work.
How many psi in a bar?: 14.50377377. A bar is exactly 100 000 Pa and a psi is exactly 6894.757293168 Pa, so the conversion follows from definitions. Note that 1 atmosphere is 14.6959 psi — the familiar 14.7 figure belongs to the standard atmosphere, not to the bar.
How do I convert kg to pounds?: Multiply by 2.204622622. The reverse factor, 0.45359237 kg per pound, is the exact definition. Note the distinction between mass and force: a kilogram converts to pound-mass this way, but a kilogram-force converts to pound-force with the same factor because both include gravity.

Value Sources

Each data column on this page is tied to the source it came from. The numbers in square brackets correspond to the table headers above.

#	Source	Type	Revision / method
[1]	Exact conversion factor defined by the unit definitions themselves	derived	A factor marked exact follows directly from the definitions of the two units and can be quoted to any number of significant figures without loss. Since the 1959 international agreement on the yard and pound, all US customary and imperial length and mass units are defined in terms of SI units exactly, so most length, area, volume and mass conversions are exact. Factors not marked exact are measured or conventionally agreed values with limited precision — treat the digits shown as the reliable extent.
[2]	NIST Special Publication 811 — Guide for the Use of the International System of Units (SI)	standard	NIST SP 811, 2008 edition — source
[3]	BIPM Le Système international d'unités (SI Brochure), 9th edition	standard	BIPM SI Brochure, 9th edition (2019)

Data Sources

Standard	Revision	What it covers on this page
NIST Special Publication 811 — Guide for the Use of the International System of Units	NIST SP 811, 2008 edition	the conversion factors and the exact/approximate distinction
BIPM SI Brochure, 9th edition	BIPM SI Brochure, 9th edition (2019)	the SI definitions and the 2019 redefinition of the base units
1959 international agreement on the yard and pound	n/a — international agreement of 1959, still in force	the exact definition of the inch, foot, yard, mile and pound

Cross-checked against:

Exact factors checked against their defining relationships — the foot is 12 × 25.4 mm, the pound-force is 0.45359237 × 9.80665 N
Seven widely published conversions held as known-value checks, including the inch, pound and psi
Reciprocal pairs verified to multiply to 1 within rounding
Approximate factors compared against the NIST SP 811 listings

Derived values — the following values on this page are calculated, not taken directly from the standard:

Value	How it is derived
Exact/approximate classification	A factor is classified exact only if it follows directly from the definitions of both units. Reciprocals of exact factors are not exact unless they terminate, which is why 25.4 mm per inch is exact while 0.03937007874 in per mm is not.

Factors marked approximate are quoted to about 10 significant figures, which exceeds the precision of the underlying definitions for values such as BTU and horsepower — more than one historical definition of each exists, and the notes state which is used. Temperature requires an offset and is covered in the first section rather than in the table.

Accuracy and use. The values on this page are compiled from the published standards and cross-checked sources listed above. Where values are derived, the derivation is stated. No warranty, express or implied, is made as to the accuracy or completeness of this information, and no liability is accepted for any loss or damage arising from its use. Engineering reference data is provided for guidance in preliminary work — before a value is used for design, fabrication or acceptance testing, verify it against the current revision of the governing standard and against your own inspection. The user assumes all risk and responsibility in connection with the use of this information.

Cite This Page

Every value on this page is traceable to the sources listed above. If you use the data in a document, paper or report, cite it as:

Plain text

Engineering Reference. "Conversion Chart." https://refmanual.com/charts/conversion-chart/ (accessed 2026-09-29). Data from NIST Special Publication 811 — Guide for the Use of the International System of Units, NIST SP 811, 2008 edition.

BibTeX

@misc{conversion-chart,
  title        = {Conversion Chart},
  author       = {{Engineering Reference}},
  howpublished = {\url{https://refmanual.com/charts/conversion-chart/}},
  note         = {Accessed: 2026-09-29},
  source       = {NIST Special Publication 811 — Guide for the Use of the International System of Units, NIST SP 811, 2008 edition}
}

RIS (EndNote, Zotero, Mendeley)

TY  - ELEC
TI  - Conversion Chart
AU  - Engineering Reference
PB  - Engineering Reference
UR  - https://refmanual.com/charts/conversion-chart/
PY  - 2026
DA  - 2026-09-29
N1  - Source: NIST Special Publication 811 — Guide for the Use of the International System of Units, NIST SP 811, 2008 edition
ER  -

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